The present invention relates to an offset blanket which is one of elements constituting an offset printing machine.
Conventionally, offset lithography is widely used as a printing method capable of producing printed matter of a high quality.
The offset lithography employs a blanket which includes a blanket cylinder and an offset blanket wound thereon.
FIG. 4 is a schematic section view of a conventional offset blanket. In FIG. 4, an offset blanket 20 includes a surface printing layer 21 made of rubber, plastic such as urethane, or the like, and a support layer 22 for supporting the surface printing layer 21. The other surface of the support layer 22 than the surface thereof for supporting the surface printing layer 21 is mounted as wound on the periphery of the blanket cylinder.
In the offset blanket 20 having the arrangement above-mentioned, a plate cylinder (not shown) transfers printing inks in a predetermined pattern to the offset blanket 20, and the printing inks thus transferred are then transferred onto printing paper supplied between the offset blanket 20 and the impression cylinder, thus achieving offset lithography.
The offset blanket 20 may be used in a special manner when carrying out UV varnishing printing, metal printing or the like with the use of ultraviolet curing inks (UV inks), oil inks or the like. That is, the offset blanket 20 is wound on the blanket cylinder, and a portion of the surface printing layer 21 is then cut and peeled off before printing is carried out. Thus, the surface printing layer 21 has a portion on which the printing inks are not to be applied. After the printing has been carried out, a stencil number is marked on the portion on which the inks have not been applied.
When cutting and peeling off a portion of the surface printing layer 21 of the offset blanket 20, such peeling can be easily made if the surface printing layer 21 has tensile strength greater than the peel strength of peeling the surface printing layer 21 from the support layer 22. When the tensile strength of the surface printing layer 21 is increased, the hardness of the surface printing layer 21 is generally increased.
However, if the hardness is increased, this not only makes it difficult to transfer the printing inks onto the surface printing layer 21, but also increases the shock exerted on the plate cylinder or the like by the surface printing layer 21. This causes the durability of the offset lithographic plate to be lowered.
To improve the peeling properties of the surface printing layer 21, the peel strength of peeling the surface printing layer 21 from the support layer 22 may be decreased. However, if the peel strength is excessively decreased, this creates the problem that the surface printing layer 21 is peeled from the support layer 22 in the course of offset lithography, provoking defective printing or the like.